1. Field of the Invention
This invention relates generally to an apparatus and method for non-destructively testing the bonding strength of the connection lines to the circuits on an integrated circuit (IC) chip. More particularly, this invention relates to the apparatus and method for non-destructively testing the bonding strength of the connection lines to the circuits on an IC chip wherein the bonding is formed by utilizing a tape automatic bonding (TAB) machine.
2. Description of the Prior Art
The reliability of the electrical connections wherein the external lines are to be securely attached to the integrated circuits (ICs) on an IC chip may often become a quality issue after a period of field operation. The main reason of this concern is caused by the fact that an effective nondestructive testing apparatus and method to determine the bonding strength of these connections are still not available.
One of the techniques for bonding the external lines to the ICs is tape automatic bonding (TAB) which uses a hot thermode to gang bond the electrical connections. Unlike wire bonding, gang bonding offers the advantage of making many bonds in a single action. Since a higher density interconnection can be achieved by TAB, it becomes popular as the inputs/outputs (I/O's) of the IC die continue to increase while the pitches between the pads are decreased. Because TAB provides greater bond strength, it offers additional advantage that a better reliability is generally achievable by applying TAB in IC manufacturing than the wire bonding.
Since many bonds are formed in a single operation by applying the gang bonding technique, an operational error or a processing parameter deviation may result in defective bonds in that operation. For that reason, more precise control has to be exercised to increase the product yield of the gang bonding process and to assure that the quality of these bonds satisfies the reliability requirements.
Defective bonds may often occur during the gang bonding process due to the temperature variations of the thermode which is frequently caused by the lack of planarity or improper design of the thermode. The bonds formed under non uniform temperature may often lack sufficient bond strength. It may also result in variations in bond height which will cause insufficient contact between the leads and the bump formed by the gang bonding process. Many weak or defective bonds may be formed by the gang bonding process which will adversely affect the reliability and performance of the final products produced by such processes if not properly tested and then rejected or corrected.
The weak bonds caused by the aforementioned problems often cause the bond strength to be less than a pull strength requirement which is approximately 10 grams. Although the pull strength formed by the gang bonding process may not be sufficient for regular field operation, the defects can not be detected by routine open-loop/close-loop electrical testing. The weak bonds however may cause reliability problems since poor lead connection may result due to the failure of these bonds during regular installation, transportation or other types of operational processes.
In order to assure that the TAB bonding formed between the electric wires and the I/O ports on an IC dice have sufficient attachment strength, a destructive test, as shown in FIG. 1, is performed. An electric lead 2 is attached to an IC dice 3 wherein the attachment is established by utilizing a TAB carrier tape 4 to form a TAB bump 5 between the electric wire 2 and the IC dice 3. A testing hook 6 is used to perform a pull-strength test to the bonding by pulling the electric wire 2 away from the IC dice 3. The usefulness of such a test is very limited for several reasons. First, the process in placing the testing hook underneath each electric wire is very time consuming. There is very limited working space around the leads on the edge for placing the testing hook underneath the leads to conduct the pull-strength test. It is virtually impossible to perform the test on the inner leads because a neighboring lead may often be destroyed in placing the testing hook underneath a lead since the distance between two neighboring leads is so small. Additionally, it is also difficult to precisely control the pulling force and to place the testing hook precisely near the attachment point. The electric wire 2 may often be destroyed without being able to obtain an accurate measurement of the attachment strength between the electric wire 2 and the IC dice 3.
Several nondestructive testing methods are proposed. The first one is to use the X-ray to detect the defective lead/bond interfaces by transmitting X-ray through these interface areas. The X-ray images of these interfaces are inspected to determine if there are non-uniform interface images which may identify the defective bonding. However, since there is no definitive and quantitative correlation between the image abnormality and the bond strength, the results obtained from the X-ray method is not very useful for systematically controlling the quality of the bonding process. Additionally, since the X-ray imaging method is very expensive and time consuming, it is not generally used for nondestructive test. The second method is to apply an ultra-sonic imaging system to obtain an ultra-sonic image of the bonding interfaces. Similarly, the defective interfaces are detected by inspecting the image to determine if a non-uniformity of the image exists in the lead/bond interface. The ultra-sonic imaging method suffers the same difficulties and limitations as the X-ray method. Additionally, a testing sample is to be immersed in the water before the ultra-sonic method is applied. It may cause a corrosion problem which may further adversely affect the reliability of the IC devices produced by this method.
Other than the destructive test by pulling the connection lines, a practical nondestructive test is not available at the present time. Long term reliability of the products can not be properly controlled due to this inability to properly test the bonding strength of these connections.
There is still a need for those skilled in the art to develop an effective nondestructive test to assure that the bonding strength of the external line connections attaching to the ICs are in compliance with the design and operation requirements.